We are in the process of determining the relative interstitial concentrations of two metabolites thought to stimulate metabolite-sensitive muscle afferents within the exercising muscle. We hypothesize that during low-level static forearm exercise the concentration of lactic and phosphate in the muscle intersitium will parallel the rise in various indices of sympathetic nerve activity. We further postulate that following a bout of forearm exercise, interstitial phosphate will correlate with indices of sympathetic nerve activity whereas interstitial lactate concentrations will not. These studies will utilize a number of state-of-the-art technologies, including microdialysis and peroneal nerve recordings of sympathetic nerve traffic, to address a key question in exercise physiology. Microdialysis provides an index of the interstitial concentration of diffusable substances. Recordings of sympathetic nerve traffic provide an 'on-line' multi-unit index of sympathetic nervous system discharge directed towards blood vessels within the skeletal muscle. Both methods have been used in humans, but they have never been used simultaneously to examine the relationship between muscle metabolism and sympathoexcitation.